vdpa/mlx5: retry VAR allocation during vDPA restart

vdpa/mlx5: retry VAR allocation during vDPA restart

VAR is the device memory space for the virtio queues doorbells,
Qemu could mmap it to directly to speed up doorbell push.

On a busy system, Qemu takes time to release VAR resources during driver
shutdown. If vdpa restarted quickly, the VAR allocation failed with
error 28 since the VAR is singleton resource per device.

This patch adds retry mechanism for VAR allocation.

Fixes: 4cae722c1b06 ("vdpa/mlx5: move virtual doorbell alloc to probe")
Cc: stable@dpdk.org
Signed-off-by: Xueming Li <xuemingl@nvidia.com>
Reviewed-by: Matan Azrad <matan@nvidia.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>